A PMR write head typically has a main pole with a small surface area at an air bearing surface (ABS), and coils that conduct a current and generate a magnetic flux in the main pole such that the magnetic flux exits through a write pole tip and enters a magnetic medium (disk) adjacent to the ABS. Magnetic flux is used to write a selected number of bits in the magnetic medium and typically returns to the main pole through a trailing loop pathway. The trailing loop generally has a trailing shield structure that arches over the write coils and connects to a top surface of the main pole layer above a back gap magnetic connection. The first trailing shield that adjoins a top surface of the write gap may have a high moment (19-24 kG) layer called a hot seed layer. A good hot seed response is required to reduce stray fields in the side shields, and in a leading shield when present. A PMR head which combines the features of a single pole writer and a double layered medium (magnetic disk) has a great advantage over longitudinal magnetic recording (LMR) in providing higher write field, better read back signal, and potentially much higher areal density.
Shingled magnetic recording (SMR) is a form of PMR and has been proposed for future high density magnetic recording by R. Wood et al. in “The Feasibility of Magnetic Recording at 10 Terabits Per Square Inch on Conventional Media”, IEEE Trans. Magn., Vol. 45, pp. 917-923 (2009). In this scheme, tracks are written in a sequential manner from an inner diameter (ID) to an outer diameter (OD), from OD to ID, or from OD and ID towards a middle diameter (MD) in a radial region of a disk in a hard disk drive (HDD). In other words, a first track is partially overwritten on one side when a second track adjacent to the first track is written, and subsequently a third track is written that partially overwrites the second track, and so forth. Track widths are defined by the squeeze position or amount of overwrite on the next track rather than by write pole width as in today's hard disk drives.
One of the main advantages of shingled writing is that write pole width no longer needs to scale with the written track width. Thus, the opportunity for improved writability and higher device yield is not restricted by pole width as a critical dimension to be tightly controlled. Secondly, adjacent track erasure (ATE) or ATI becomes less of an issue because tracks are written sequentially in a cross-track dimension and only experience a one time squeeze from the next track.
For both conventional magnetic recording (CMR) and shingle magnetic recording (SMR), continuous improvement in storage area density is required for a PMR writer. A write head that can deliver or pack higher bits per inch (BPI) and higher TPI is essential to the area density improvement. The main pole and hot seed layer in the first trailing shield are usually comprised of high moment (19-24 kG) material while the side shields, and remainder of the trailing shield structure are made of low moment (10-19 kG) materials. If writeability can be sustained, the main pole size must shrink, and a thinner write gap at the main pole trailing (top) surface and a narrower side gap adjoining the main pole sides in the cross-track direction are preferred for better track field gradient (Hy_grad, BPI) and cross-track field gradient (Hy_grad_x, TPI), respectively. Although a side shield made entirely of low moment (low Ms) material exhibits low far side ATI, severe side shield tip saturation degrades TPI and is a major obstacle to further decreasing the side gap (SG) dimension for area density control (ADC) improvement.
Therefore, a new shield design is needed to minimize side shield tip saturation and maintain low ATI on the far side in order to enable higher TPI and a smaller side gap for TPI capability to at least 500K/in for CMR and at least 700K/in for SMR. A new design must take into account that the geometries and dimensions of the side shield are key to both overwrite (OW) and bit error rate (dBER) performance in hard disk drives (HDD).